Gas turbine engines operate to produce mechanical work or thrust. Specifically, land-based gas turbine engines typically have a generator coupled thereto for the purposes of generating electricity. The shaft of the gas turbine is coupled to the generator. Mechanical energy of the shaft is used to drive a generator to supply electricity to at least a power grid. The generator is in communication with one or more elements of a power grid through a main breaker. When the main breaker is closed, electrical current can flow from the generator to the power grid when there is a demand for the electricity. The drawing of electrical current from the generator causes a load to be applied to the gas turbine. This load is essentially a resistance applied to the generator that the gas turbine must overcome to maintain an electrical output of the generator.
Under normal operating conditions, the main breaker is in a closed position and electrical output from the generator is supplied to an electrical grid. However, when the main breaker is opened suddenly, as would occur during a full load rejection, the generator load on the shaft of the gas turbine engine drops to zero. Due to the inertia of the gas turbine engine under a load condition, the shaft speed can increase rapidly when the load is suddenly removed. As a result, the engine can overspeed, if care is not taken to control the unloading of the gas turbine engine. In certain configurations, a secondary breaker is used in addition to the main breaker. The secondary breaker is inserted in the power grid between a plant electrical network and the power grid. In some cases when the secondary breaker is opened suddenly, as would occur during a partial load rejection, the generator load on the shaft of the gas turbine drops sharply. However, the generator is still supplying electricity to the plant electrical network and hence the load on the shaft is highly reduced but remains greater than zero load. As for a full load rejection, this can result in engine overspeed and must be controlled to prevent engine damage. An overspeed can also occur when the breaker trips as a result of detecting a loss of flame in the combustor.
In order to control the engine speed when partially or fully unloaded, the fuel flow is typically reduced and redirected within the combustion system. Reducing the fuel flow will reduce the operating temperature and output of the turbine section, which will in turn, reduce the engine speed. However, care must be exercised when reducing and redistributing the fuel flow so as to maintain a stable flame in the combustor while also returning the engine to a condition capable of being reloaded as soon as possible, so as to minimize any time that electrical generation is interrupted in case of a full load rejection.